An increasing number of HVAC units are using variable speed compressor technology to improve efficiency, comfort, and reliability. Variable speed systems typically work by connecting a motor control drive to the compressor and then connecting the line input power from the utility to the drive. The drive uses frequency modulation to adjust power output of the compressor motor enabling it to speed up or slow down according to the heating or cooling load in the home. This ability to modulate compressor capacity enables many of the advantages of variable speed technology including improved efficiency and comfort.
Generally, HVAC systems using variable speed compressor technology require a system control capable of communicating the indoor air temperature and outdoor air temperature for controlling the compressor. Generally, the communicating capability is a part of a proprietary protocol; thus, increasing the number of components of the system and increasing costs. There is therefore a need for a system and method to control a variable speed compressor at minimal costs.